fix(storage-vercel-blob): properly handle alwaysInsertFields and add comprehensive integration test suite with a vercel blob emulator

[r1tsuu]

This PR:

• Adds Vercel Blob Emulator ghcr.io/payloadcms/vercel-blob-emulator:latest from https://github.com/payloadcms/vercel-blob-emulator to our docker-compose.yml (same purpose as with localstack, azure-storage etc.)
• Adds comprehensive integration test suite for the vercel blob adapter (before we did not have any), including client uploads. Pretty much copy of our tests for S3, just adapted for vercel SDK instead of AWS S3.
• Updates the SDK (@vercel/blob) to 2.3.1
• Fixes an issue with alwaysInsertFields - previously we did not handle this property at all - the copied tests from S3 caught that. Similar to fix(storage-gcs): correctly handle alwaysInsertFields #15541
• Adds ability to override baseUrl with STORAGE_VERCEL_BLOB_BASE_URL env variable - required for the emulator so requests don't go to *.blob.vercel-storage.com.

The emulator is good enough for local developing if you want to avoid locally either switching to local disk storage conditionally or using a vercel blob token.

[github-actions]

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